Keywords

1 Recycled Materials and Prefabricated Buildings

In recent years, countless new buildings have sprung up around the world, while a large number of abandoned buildings have been demolished, resulting in a huge amount of construction waste. The volume of construction waste in China is huge, and according to analysis, the total amount of waste concrete in the construction waste generated every year in China is continuing to grow at an annual rate of 8%. But after being treated, waste concrete becomes recycled material and can re-enter the construction process, starting a new life cycle. Some prefabricated buildings [1,2,3] have begun to attempt to use components produced from recycled materials, further digesting construction waste. However, compared to natural aggregates, recycled aggregates still have significant differences. Recycled coarse aggregates have characteristics such as uneven material distribution, high porosity, poor adhesion and frost resistance, which undoubtedly affect the mechanical properties of concrete [4].

2 Recycled Building Materials

Bending strength refers to the ultimate breaking stress of a material when subjected to bending moments per unit area. The flexural strength is also an important parameter for the mechanical properties of concrete. Research in the academic community has shown that the flexural performance of recycled concrete is slightly lower than that of ordinary concrete [5, 6]. In the experiment, Xiao Bei [7] showed a continuous decreasing trend in the flexural strength of concrete as the amount of recycled materials increased. The reduction in substitution rates of 25%, 75%, and 100% was about 25.21%, 35.14%, and 40.31%, respectively. Parthiban Kathirvel [8] found through research results that the flexural performance of recycled concrete is relatively poor compared to ordinary concrete. Rui Rao [9] found that the performance of recycled concrete can be improved by adding other materials. Overall, the flexural strength of recycled concrete is lower than that of natural concrete, but it can be improved by adding other materials. Diamond sand is a type of high-strength and wear-resistant stone material that can be used as concrete aggregate. Moreover, China has abundant reserves of diamond sand. Adding diamond sand can help improve the flexural strength of recycled concrete, which is of great benefit to the application and promotion of recycled concrete.

However, there is relatively little research on the addition of diamond sand to concrete both domestically and internationally [10]. This study focuses on recycled concrete, changes the content of diamond sand added, and examines the changes in the flexural strength of recycled concrete, in order to help improve the flexural strength of recycled concrete by adding diamond sand.

3 Experimental Research

By testing the bending resistance of concrete, the author finds out the amount of emery that can meet the strength requirement of prefabricated concrete structure, so as to achieve the purpose of environmental protection, economy and practical application of prefabricated concrete construction.

3.1 Test

(1) Raw materials. Cement: PO42.5 ordinary Portland cement. Coarse aggregate: natural coarse aggregate particle size is 5–31.5 mm, the apparent density is 2563 kg/m3; The regenerated coarse aggregate has a particle size of 5–31.5 mm and an apparent density of 2489 kg/m3. Fine aggregate: ordinary natural river sand fineness modulus 2.47, particle size < 5 mm; Machine-made sand has a fineness modulus of 3.32 and a particle size of < 5 mm. Carborundum: the particle size is 0 ~ 2 mm, the apparent density is 3012 kg/m3. Admixture: Grade I fly ash, fineness 8.0%; Grade S95 mineral powder with a density of 2.9 g/cm3. Admixture: composite water reducing agent, water reducing rate of 22% ~ 24%, gas content of 3.9%.

(2) Mix ratio. In this experiment, the mix ratio is based on C30 concrete, the replacement rate of recycled aggregate is 30%, and the addition amount of emery is 8%, 16%, 24% and 32%, respectively. The mix ratio design of each group of specimens is shown in Table 1.

Table 1. Mix ratio design of specimens
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(3) Environmental conditions: Room temperature physics laboratory

Test steps: A. Clean and level the surface of the sample to prevent any impact on the test data due to differences in workmanship between each group of test blocks.

B. Mark the processed test block with lines according to Fig. 1 to ensure accurate placement of the test block.

C. Adjust the position of the testing machine support to ensure accurate positioning.

D. Turn on the testing machine to ensure that the collet contacts the test block smoothly and evenly.

Start the experiment and observe the changes in the sample, and record the data after reaching the ultimate load.

Fig. 1.
figure 1

Schematic diagram of specimen loading

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3.2 Flexural Test Design

(1) The sample is 150 mm × 150 mm × 550 mm standard trabecular sample, and 3 identical samples of each mix ratio are prepared, a total of 18. The test was carried out on the MTS microcomputer controlled electronic pressure test machine, and the three-minute loading method was adopted. Take the specimen out of the curing room and polish the place in contact with the support of the testing machine and align it with the support, so that the contact between the specimen and the support is stable. First manually debug the loading point as close as possible to the specimen, and then at the loading speed of 0.05 MPa/s, so that the testing machine began to work.

(2) The experimental results of each group are shown in Table 2. As can be seen from Table 2, the failure process of natural concrete, recycled concrete and emery modified recycled concrete is that cracks appear at the bottom of the pure bend section of the specimen at the beginning. With the increase of load, the crack width also increases, and the specimen suddenly breaks. At this time, the failure load has been reached and the specimen has obvious brittle failure characteristics.

Table 2. Results of bending strength test
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3.3 Result Analysis

(1) Damage state analysis. According to the analysis of the failure section, the failure interface of natural concrete is relatively smooth and clean, and near the middle line of the pure bend section of the specimen, the cross section is composed of coarse aggregate cross section and cement mortar cross section, and it can be seen that the internal honeycomb porosity is less. The failure interface of recycled concrete is relatively rough and uneven, although it is in the pure bend section, it is distributed on both sides, which is because the surface condition of the coarse aggregate in the recycled concrete is relatively complicated, there is non-uniformity caused by the interface zone of the old and new cement mortar, and there are holes inside. The damage interface of recycled concrete mixed with emery is between the two, because the addition of emery improves the cavity phenomenon and makes the internal distribution of recycled concrete more uniform.

(2) Flexural strength analysis. According to the analysis of the bending strength test results, the bending strength of concrete does not decrease significantly with the addition of 30% recycled aggregate, which is somewhat different from the results in the literature. The reason may be that the recycled coarse aggregate has high porosity and water absorption, resulting in poor adhesion to the section of cement mortar. Therefore, there are more holes in the recycled concrete, and the compacted degree is low, which affects the bending strength. The bending strength of emery concrete increases with the increase of the amount of emery, which shows that under the appropriate amount of emery, it can make up for the strength defects caused by recycled aggregate, and with the increase of the amount of emery, the bending strength also continues to increase, and can achieve the bending strength of the same mix ratio of natural concrete. (See Figs. 2 and 3).

Fig. 2.
figure 2

Flexural strength of specimens in each group

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Fig. 3.
figure 3

Flexural strength ratio of each group

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From the above results, it can be seen that the addition of diamond sand can effectively improve the flexural performance of recycled concrete to a certain extent. However, it was also found from the experimental results that after the diamond sand content reached 16%, the flexural strength of the sample was significantly improved. However, when the diamond sand content increased subsequently, the improvement in flexural strength was not significant. Moreover, the density of diamond sand is higher than that of river sand, making it difficult to disperse in cement slurry and prone to settlement. The more the amount added, the more severe the settlement phenomenon becomes, resulting in uneven composition of the upper and lower parts of the sample. At the same time, diamond sand settlement also leads to a certain amount of capillary pores inside the concrete, reducing the compactness of the concrete. This is also the reason why the rate of improving the flexural strength of recycled concrete slows down after increasing the content of diamond sand.

4 Conclusion

From the above discussion, the following conclusions can be drawn:

  1. (1)

    The addition of recycled materials significantly reduces the flexural strength of concrete.

  2. (2)

    Diamond sand can effectively improve the cavity phenomenon caused by the weak bonding force between the new and old mortar interface areas inside recycled concrete, thereby enhancing the flexural performance of recycled concrete.

  3. (3)

    When the replacement rate of diamond sand is 16%, the flexural performance of recycled concrete is similar to that of ordinary concrete.

5 Limitation

Due to raw material reasons, the mechanical properties of recycled concrete are inferior to those of ordinary concrete, which is one of the reasons why recycled concrete has not been widely promoted. Adding diamond sand can improve the flexural performance of recycled concrete, and the cost of diamond sand is low, which has great significance for later research and application. However, at present, research on the addition of diamond sand to recycled concrete is very limited, and more related research can be conducted in the future to lay the foundation for the promotion and application of recycled concrete.